Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen and catalyst to lower molecular weight hydrocarbons. Depending on the desired output, the hydrocracking zone may contain one or more beds of the same or different catalyst. Hydrocracking is a process used to crack hydrocarbon feeds such as vacuum gas oil (VGO) to diesel, including kerosene, and gasoline motor fuels.
Mild hydrocracking is generally used upstream of a fluid catalytic cracking (FCC) unit or other process unit to improve the quality of an unconverted oil that can be fed to the downstream unit, while converting part of the feed to lighter products such as diesel. As world demand for diesel motor fuel is growing relative to gasoline motor fuel, mild hydrocracking is being considered for biasing the product slate in favor of diesel at the expense of gasoline. Mild hydrocracking may be operated with less severity than partial or full conversion hydrocracking to balance production of diesel with the FCC unit, which primarily is used to make naphtha. Partial or full conversion hydrocracking is used to produce diesel with less yield of the unconverted oil which can be fed to a downstream unit.
Due to environmental concerns and newly enacted rules and regulations, saleable diesel must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
Integration of hydroprocessing units may involve situations where one unit runs at a higher pressure than the other unit. For example, a hydrocracking unit typically runs at a higher pressure than a hydrotreating unit. Hydrogen must be supplied at different pressures. Excess hydrogen is recycled through a recycle gas compressor dedicated to each hydroprocessing unit.
There is a continuing need, therefore, for improved methods of producing more diesel from hydrocarbon feedstocks than gasoline. Such methods must ensure that the diesel meet increasingly stringent product requirements. There is also need for improved methods of supplying hydrogen to separate process units at different pressures.